The 3GPP long term evolution (LTE) Rel. 8 and later standards define various transmission schemes for communication systems equipped with multiple transmit antennas and multiple receive antennas. Multi-user multiple input multiple output (MU-MIMO) is a set of advanced multiple-input and multiple-output, technologies where the available antennas are spread over a multitude of independent access points, such as base stations, and independent terminals—each having one or more antennas. To enhance the communication capabilities of all terminals, MU-MIMO applies an extended version of space-division multiple access (SDMA) to allow multiple transmitters to send separate signals and multiple receivers to receive separate signals simultaneously in the same band.
One transmission scheme is downlink beamforming, where multiple antennas at the enodeB (eNB), also referred to herein as a base station (BS), transmit to multiple user equipments (UEs) using the same time-frequency resource. The LTE Rel. 10 standard defines a codebook for up to eight transmit antennas per BS.
Beamforming is a signal processing technique involving directional signal transmission or reception. This is achieved by combining elements in a phased array in such a way that signals at particular angles experience constructive interference while others experience destructive interference. Beamforming can be used at both the transmitting and receiving ends in order to achieve spatial selectivity. In a centralized radio access network (C-RAN), multiple transmit antennas across multiple BS can be utilized to jointly beamform over a large geographic area. The C-RAN system has the potential to provide a significant gain in spectral efficiency and better network coverage. However, co-channel interference, if not efficiently controlled or cancelled, limits these advantages.
Channel state information (CSI), which refers to known channel properties of a communication link, can be used to control co-channel interference. This information describes how a signal propagates from the transmitter to the receiver and represents the combined effect of, for example, scattering, fading, and power decay with distance, making it possible to adapt transmissions to current channel conditions. CSI is typically estimated at the receiver and quantized and fed back to the transmitter.
A transmit beamforming algorithm can preemptively remove or reduce the interference in a MU-MIMO system. Block diagonalization and zero-forcing are two well-known beamforming techniques that utilize the downlink channel state information (DL-CSI) of users to perform linear precoding of their signals before transmission in order to cancel the co-channel interference. The performance of these beamforming techniques rely on the accuracy of the DL-CSI. Amplitude hardware-induced distortion that is introduced by BS transmit chains and UE receive chains, however, prevents an accurate determination of DL-CSI.
Accordingly, there exists a need for methods, systems, and computer program products for calibrating amplitude hardware-induced distortion in an LTE communications system.